Traceback (most recent call last):

File "python", line 36, in

File "python", line 28, in grades_std_deviation

TypeError: unsupported operand type(s) for ** or pow(): 'list' and 'float'

So I have been frustrated with the review section of 9/9. I'm pretty sure my code is correct and the error didn't show up on the previous exercise even though it was the same code. If anyone could tell me if I'm doing something wrong that would be great.

```
grades = [100, 100, 90, 40, 80, 100, 85, 70, 90, 65, 90, 85, 50.5]
def print_grades(grades):
for grade in grades:
print grade
def grades_sum(grades):
total = 0
for grade in grades:
total += grade
return total
def grades_average(grades):
sum_of_grades = grades_sum(grades)
average = sum_of_grades / float(len(grades))
return average
def grades_variance(scores):
average = grades_average(scores)
variance = 0
for score in scores:
variance += (average-score) ** 2
return variance / len(scores)
print grades_variance(grades)
def grades_std_deviation(variance):
return pow(variance, .5)
variance = grades_variance(grades)
print grades_std_deviation(variance)
print_grades(grades)
print grades_sum(grades)
print grades_average(grades)
print grades_variance(grades)
print grades_std_deviation(grades)
```